A positive balance for phosphate is essential to the process of growth. Clearance studies indicate that the kidney contributes to this positive external balance by maintaining a high fractional reabsorption of phosphate (TRP) in relation to the plasma concentration of this substance. The factors accounting for the relativey high TRP in the growing mammal have been explored to detail. It is likely that differences in intrinsic renal transport process between immature and mature animals play a role in this regard. In addition, evidence supports the supposition that extrarenal factors which are active during development and dormant at maturity serve to further modify the kinetics of renal phosphate handling. Differences in the contransport of phosphate with sodium and in its interaction with glucose uptake fall into the former category. Extrinsic factors which may have a unique effect on renal phosphat handling by the kidney of the growing animal include total body phospatek growth hormone and PTH. The relationship between PTH ad cyclic-AMP, its intracellular messenger, may also be different in growing and in mature animals. In order to study the details of this complex system for phosphate homeostasis it is necessary to design experiments that allow indepent manipulation of each of the many variables. Comparisons of the effects of various manipulations on phosphate transport kinetics of brush border microvesticles prepared from guinea pigs of various ages will permit us to assess the renal role played by each control mechanism at different stages of postnatal development.